Similar flowmeters with this type of structure, are described in the U.S. Pat. No. 3,640,133, U.S. Pat. No. 3,690,171, Japan Patent Official Disclosure No. 48-54962, Japan Patent Official Disclosure No. 53-77558 and Japan Patent Official Disclosure No. 59-184822.
Because this type of flowmeter functions to blow out the measured fluid from the nozzle, as described above, in order to generate the fluid vibration, it is essential that the flowrate of the fluid being blown out of this nozzle be stable in proportion to the flowrate, and a measurement error occurs if the flow lacks this stability.
The flowrate of fluid being blown out of the nozzle is governed greatly by the shape of the upstream side of the nozzle together with the shape of nozzle. For example, if there should be anything hindering a smooth flow in a flow channel on the upstream side of nozzle, a vortex flow appears to the fluid flowing into the nozzle, and this vortex flow exerts a great influence on the flowrate, by changing the flowrate.
Therefore, as it can be understood from the aforesaid officially know fluidic flowmeters, such a device has been made such that a straight flow channel is formed on the upstream side of nozzle for making a vortex flow unlikely to appear.
Forming a straight flow channel on the upstream side of nozzle in this way is possible in an experimental laboratory experiment room or installation conditions with plenty of room. However, if a compact size is required for installation conditions like the gas meter of city gas being supplied to general households, for example, through piping, it is impossible to form a sufficient straight flow channel on the upstream side of nozzle. Moreover, in the case of this type of gas meter, the valves and governors are frequently installed on the upstream side of nozzle, and there also exist the bends of piping or the land offset of joints, all of which become factors causing vortexs in the gas flow and exerting a detrimental influence on the stability of flowrate.